Shell manufacturing method for precision casting

ABSTRACT

The preparation of a composite shell mold for precision casting wherein portions of the shell mold of large dimension and complicated shapes are formed by the lost wax process while other portions are formed by conventional molding or permanent molding techniques for joinder into a composite mold.

United States Patent Ingalls et al.

[ 1 June 13, 1972 [54] SHELL MANUFACTURING METHOD FOR PRECISION CASTING[72] Inventors: John E. Ingalls, Pentwater, Mich; Charles Yaker, Summit,NJ.

[73] Assignee: Howmet Corporation, New York, NY. [22] Filed: Sept. 8,1969 211 App]. No: 855,941

[52] U.S. Cl ..164/26, 164/34, 164/27, 164/ 129 [51] Int. Cl. ..B22c9/00 [58] Field of Search ..164/24, 25, 26, 129, 35, 36, 164/77, 129,137, 23, 30, 31, 27, 29, 28

[56] References Cited UNITED STATES PATENTS 2,789,331 4/1957 Dietert164/27 3,008,204 1 1/1961 Herman ..164/3O X 3,010,166 11/1961 Skoning..164/29 3,059,296 10/1962 North ..164/23 3,472,310 10/1969 Ernest et a11 64/29 X R26,495 12/1968 Watts et al. 164/26 3,424,227 H1969 Watts etal ..164/36 2,845,669 8/1958 Hackett et a1. ..164/137 3,503,435 3/1970Dunlop ...164/26 X 3,429,358 2/1969 Tingquist et al. ..164/26 PrimaryBraminer--.l. Spencer Overholser Assislan! Examiner-V. K. Rising Au0rnev-McDougall, Hersh, Scott & Ladd [5 7] ABSTRACT The preparation of acomposite shell mold for precision casting wherein portions of the shellmold of large dimension and complicated shapes are formed by the lostwax process while other portions are formed by conventional molding orpermanent molding techniques for joinder into a composite mold.

16 Claims, 11 Drawing Figures PATENTEDJUH 1 I972 3.669177 sum 1 OF 2 IINVENTORS John f. In ails Charles y aker Y difys SHELL MANUFACTURINGMETHOD FOR PRECISION CASTING This invention relates to the process forprecision casting of metal parts of large dimension and of complexshapes and it relates more particularly to the casting of parts of largedimension and complex shapes as an integral unit.

To the present, as described in the issued patent of Operhall et al.,U.S. Pat. No. 2,961,751, integral castings of complex shapes have beenprecision cast of super alloys by the assembly of a plurality of heatdisposable patterns, conforming to the parts to be molded, into acluster having the necessary sprues and runners, similarly formed ofheat disposable material. The cluster is then processed through a seriesof alternating dip coats and stucco coats of ceramic materials to buildup a ceramic shell about the exposed surfaces of the cluster. Aftersufficient drying or setting of the applied dip and stucco coats, thecluster with the shell of ceramic material formed thereon is exposed tohigh temperature to melt and to burn out the heat disposable material,leaving a ceramic shell containing spaces formerly occupied by the heatdisposable patterns and inter-connected by channels formerly occupied bythe gates and runners. The ceramic shell is further heated to cureand/or preheated prior to pouring the molten metal therein. The moltenmetal flows through the gates and runners to fill the cavities formerlyoccupied by the heat disposable patterns. After cooling to solidify themetal, the ceramic shell is broken away to expose the integral castingfrom which the metal parts are separated and cleaned. The describedprecision casting process embodies the principles of the lost waxprocess but with considerable improvement for precision casting ofcomplicated shapes of super alloys and metals which otherwise aredifficult to process.

The described precision casting process, as it is practiced today,requires considerable labor for use in the fabrication of a pattern andthe assembly of patterns into a cluster having a suitable design andarrangement for use in a ceramic shell from which molded parts can besecured in high yield. Further, the size or dimension of the parts thatcan be joined into a single pattern or cluster for producing an integralcasting is limited, such that it is difficult to cast parts such as anentire wheel having a plurality of turbine blades formed integrally withthe hub of the wheel and extending outwardly radially therefrom inuniformly spaced apart relation and with sufficient assurance that theblades will be completely filled for an acceptable composite castingcapable of meeting rigid specifications. The turbine wheel described ismerely illustrative of the type of structure having a size and shapewhich is difficult to cast by current precision casting techniques.

It is an object of this invention to provide a process for thepreparation of composite molds from which integral castings of largedimension and/or shapes can be produced in high yields by the precisioncasting process; in which the composite mold is formed of separate partscapable of being assembled together to provide a composite mold in whichthe molten metal can be poured to produce an integral metal casting; inwhich separate parts of the mold are capable of fabrication by moreefficient and effective techniques for more accurate and more economicalproduction of the parts with corresponding increase in yield of metalcastings produced therefrom; and in which such parts can be assembled toproduce shells from which castings of large dimension or complicatedshapes can be produced.

These and other objects and advantages of this invention willhereinafter appear and, for purposes of illustration, but not oflimitation, an embodiment of the invention is shown in the accompanyingdrawings, in which FIG. 1 is a perspective view of a molded pattern ofthe multi-bladed air foil for use in the casting of a multi-bladedturbine wheel;

FIG. 2 is a perspective view of the assembly of a number of the air foilpatterns of FIG. 1 in a roll assembly;

FIG. 3 is a sectional elevational view of a portion of the roll assemblyof FIG. 2 showing the ceramic shell formed about the patterns;

FIG. 4 is a sectional elevational view similar to that of FIG. 3 showingthe air foil shell after removal of the pattern of heat disposablemetal;

FIG. 5 is a sectional elevational view similar to that of FIG. 4 showingthe removal of the portion of shell interconnecting the axially alignedair foil shells for separation into air foil sectrons;

FIG. 6 is a perspective view of the separated shell for casting thewheels;

FIG. 7 is a perspective view of a disc plate molded of ceramic material;

FIG. 8 is a perspective view of the pouring cup molded of ceramicmaterial;

FIG. 9 is a sectional view through a portion of the assembly showing thedisc plate and pouring cup joined to the air foil shell to form thecomposite shell mold;

FIG. 10 is a perspective view of the integral turbine wheel cast in theshell mold of FIG. 9; and

FIG. 11 is a view similar to that of FIG. 3 showing the use of separatordiscs between pattern parts.

The invention will be described with reference to the preparation of acomposite mold for casting an integral turbine wheel having a centralhub portion 10 with a plurality of air foil blades 12 extending radiallyoutwardly from the periphery of the hub portion. It will be understoodthat the concepts described for the fabrication of the composite moldfor casting the turbine wheel are applicable also to the preparation ofcomposite molds of multiple parts for the cast ing of other products oflarge dimension and/or complicated shapes.

Furthermore, since the invention is addressed primarily to the elementsof the shell mold, their manufacture and assembly to produce thecompleted composite mold into which the metal may be cast, detaileddescription will not be given of the composition from which the patternsare formed or of the dip coat and stucco composition used to form theceramic shell about the various segments of the mold. Compositionssuitable for use in the fabrication of the pattern parts and the shellare disclosed in numerous patents on precision casting. For thispurpose, reference may be made to the aforementioned Operhall et al.patent or to U.S. Pat. Nos. 2,441,695, 3,196,506 and 3,132,388.

Suffice it to say that the pattern parts are formed of a disposablematerial to enable removal from the mold after the ceramic shell hasbeen formed thereon. The pattern parts are preferably formed of a heatdisposable material, such as wax and more preferably a plastic materialwhich can be molded by conventional molding techniques for accuratereproduction of the part to be molded, such as polystyrene, polymethylmethacryate.

Referring now to the drawings, FIG. 1 illustrates the pattern conformingto the air foil having a central rim portion 20, in the form of acylindrical section, with a plurality of blades or vanes 22 extendingoutwardly radially from the outer peripheral surface of the rim incircumferentially spaced apart relation. The rim section and theseparate blades can be separately formed and assembled by cementing theindividual blades to the rim. It is preferred, however, that the entireassembly be molded as an integral unit of a suitable thermoplasticmaterial, such as polystyrene, polyethylene, polyvinyl acetate, vinylchloride vinyl acetate copolymer and the like, whereby the assembly canbe produced at much lower cost with assured accuracy in the location andarrangement of the blades.

As illustrated in FIGS. 2 and 3, a number of such integrally molded airfoils sections 24 are mounted in side by side, axially aligned relationon a supporting drum 26 for rotational movement as a unit to form aceramic :shell 28 about the assembled, laterally spaced apart air foilpatterns. For this purpose, each pattern portion 24 is mounted on asupporting sleeve 30 dimensioned to be received in fitting relationshipwithin the rim 10, with each sleeve 30 being formed at its ends beyondthe rim with an axial extension 32 having an annular flange 33 ofrectangular cross section with means for securing one section toanother, such as forming the flange 33 with an axial tongue 34 andgroove 36 arranged for inter-fitting with tongues and grooves in theends of adjacent sleeve members for the assembly of two or more sleevesections into an interconnected unit. The sleeve sections are mounted onthe supporting drum 26 having an axle 38. Clamping discs 40 are axiallyslidable on the axle into engagement with the outermost sleeve sectionto clamp the sleeve section in their assembled relationship on the drum.

The axles are mounted for turning movement, as indicated by the arrow inFIG. 2, to effect rotational movement of the patterns mounted on thedrum. Also, the axle 38 is supported for movement in a horizontaldirection between a clip tank 42 and a stucco hood 44 offset one fromthe other and for movement in the vertical direction to enable thepattern assembly to be lowered for immersion of the patterns to a levelabove the rim portion into a bath 46 of the dip coat composition to coatthe entire exposed surfaces of the pattern with the dip coat compositionas the assembly is slowly turned.

When the pattern has been rotated during immersion for at least one andpreferably a number of complete revolutions to insure uniform wetting ofthe pattern cluster, the entire assembly is raised from the dip coatcomposition and displaced to a position below the hood 44 whilecontinuously revolving the assembly to maintain a uniform coating of thedip coat composition on the pattern surfaces. Stucco is sprayed onto thepattern wet with the dip coat composition while rotational movement iscontinued uniformly to stucco the pattern with ceramic material, such asparticles of alundum, zircon and the like. It will be understood that,instead of moving the drum with the assembled patterns between the dipcoat station and the stucco station, the drum with the assembledpatterns can be mounted for rotational movement in a single locationwhile the tank 42 containing the dip coat composition is brought intoposition beneath the assembly and raised for partial immersion of thepatterns into the dip coat composition and the stucco hood is moved, inturn, into position over the assembly wet with the dip coat compositionfor applying the stucco 48 to the wet surfaces of the pattern.

The described operations are repeated for a number of cycles, preferablywith interim drying or setting until a ceramic shell 28 of the desiredthickness has been built up about the assembled patterns, as illustratedin FIG. 3.

Thereafter, the drawn 26 and the sleeves 30 are removed and thecomposite formed of the patterns and ceramic shell is introduced into achamber heated to a temperature above the melting point temperature ofthe material making up the pattern, and preferably to a temperaturewithin the range of l,500 to 2, 1 F. At such temperature, heat transfersrapidly through the ceramic shell to reduce the pattern material toflowable or combustion temperature for flash removal of the patternmaterial. This leaves the ceramic shell with the hollow rim andconnecting bladed portions corresponding in shape and dimension to theair foil wheel. Exposure to elevated temperature for a few minutes issufficient completely to remove and burn out the pattern material and tocure the ceramic material making up the formed shell 50.

The formed shell is processed to separate the air foil segments one fromthe other. For this purpose, the ceramic material in the raised portions52 intermediate the segments is removed, as by means of a wire brush 54or grinding wheel, as the assembly is rotated until the annular rimsection 56 is reached cleanly to separate the segments one from theother without interference with the surfaces defining the mold sectionand to provide a surface 56 of predetermined contour and shape forreceiving other parts in fitting relationship to form the completedshell for molding, as will hereinafter be described.

A disc plate 60, dimensioned to span one side of the opened end of theair foil mold, is separately formed of ceramic material. In thepreferred practice of this invention, the disc plate 60 is formed bycompression molding, or by slip casting, or by injection moldingdirectly of ceramic material to the shape and dimension for receipt infitting relationship on one end of the shell mold 50 of the air foilsection. If the disc plate 60 is of a contour that militates against theuse of conventional molding techniques of the type described, the discplate can be formed as a shell about a pattern of heat disposablematerial by the alternating cycles of clip coating, stuccoing and dryingto form a shell about the pattern followed by removal of the pattern inthe manner described to provide the disc plate of ceramic material.

It is preferred to produce the disc plate by compression or injectionmolding or by slip casting, since such techniques provide a low costmold part which can be molded directly to the desired shape and toprovide a mold surface having greater accuracy and smoothness.

Instead of providing the disc plate in the form of a cast or moldedceramic material, the disc plate or its equivalent can be provided inthe form of a permanent member capable of repeated use as an element ina composite mold. For this purpose, the disc plate can be formed ofgraphite or of metal preferably cored for the passage of a coolantliquid, such as water, therethrough. Such construction with a cooleddisc plate finds further utility for grain size and orientation controlas described in U.S. Pat. No. 3,248,764.

The disc plate 60 is molded with outer end portions 62 having an annulargroove 64 in its inner-face with an axial portion 66 dimensioned to bereceived in telescoping relation within the annular edge portion 68 ofthe air foil mold and a radial portion 70 which abuts against the radialsurface 72 in the edge of the air foil mold formed by the annular flange33 thereby to permit an interfitting relationship between the air foilshell 50 and the disc plate 60. The mold sections are joined in theirassembled relation by a suitable ceramic adhesive, such as sodiumsilicate, metal phosphate and the like with or without ceramic filler,to effect a bonded relationship between the mold segments. Instead, usecan be made of a ceramic clip to hold the parts together.

The pouring spout 74 is similarly molded of ceramic material or byforming a ceramic shell about a pattern of disposable material. Thepouring spout 74 is similarly adapted to be joined onto the other end ofthe air foil shell opposite the disc plate 60. For this purpose themolded spout is formed with an annular groove 76 in its inner-faceoutwardly of the mold surface having a flat axial portion 78 and a fiatradial portion 80 adapted to interfit with the inner portion 82 of therim and the outer edge 84 of the air foil section to effect an assembledrelationship therebetween.

The assembled mold parts are heated to cure the ceramic material andadhesive joints to form a composite, cured ceramic shell mold into whichthe molten metal can be poured.

In pouring, the ceramic shell is preheated to elevated temperature,approximating the temperature of the molten metal poured into the mold.The molten metal is poured into the mold while resting on the disc platewith the molten metal being introduced through the pouring spout in anamount to fill the mold cavity at least to the base of the pouringspout. The metal may be poured into a stationary shell or the shell maybe rotated about its axis during the pouring of the metal centrifugallyto displace the molten metal into the outermost recesses of the moldcavity thereby to insure complete filling of the bladed air foilsections prior to the inner body or hub portion of the mold. Thus acomplete integral casting is secured of the bladed turbine wheel in asingle molding operation.

By way of modification, the pattern sections 24 can be mounted on thesupporting drum 26 in side by side relationship with a separator member25 between pattern sections, preferably in the form of a thin, flexibledisc member of rigid or thin paper or plastic material. The disc memberis dimensioned to extend beyond the adjacent portions of the patternsections by a substantial distance so as to enable the disc member to bebroken away after the ceramic materials have been applied to build upthe shell mold of the desired wall thickness. Thus the separator member25 will maintain the shell parts in easily separated relation on theircylindrical support.

In the event that use is made of such disc shaped separating membersbetween pattern parts, the interfitting tongue and groove arrangementpreviously described can be replaced by flat walls which are adapted tobe joined to the walls of other shell parts making up the compositeshell mold with the application of adhesive to the surfaces to bejoined.

By way of still further modification, especially in connection with theuse of such disc shaped separators or flanged separators of the typepreviously described, it will be understood that separation can beeffected between the aligned pattern parts and the composite shell moldformed thereabout before removal of the pattern material. Thus it is theseparated pattern with the ceramic shell that is subjected to heattreatment to effect pattern removal and cure of the ceramic shellmaterial. This latter procedure of subdividing the composite shell priorto pattern removal is often preferred where the drum support mightinterfere with the ease or efficiency of heat treatment or with theremoval of pattern material from within the ceramic shell. Thesubdivision is also preferred from the standpoint of contamination sincesubdivision of the ceramic shell subsequent to removal of the patterncan permit ceramic material to enter into the mold cavity duringgrinding to subdivide the composite shell.

Similarly, pattern removal can be effected by heat treatment after allof the shell parts have been assembled to form the composite shell moldwhereby pattern removal for all of the parts can be effected in the oneand same heat treatment to produce a composite and cured shellstructure.

The described mono-rolled cluster of patterns can be carried out as acontinuous operation. For this purpose, the drum roll with the patternsections mounted thereon can be supported on a conveyor which isoperatively engaged during movement to effect rotational movement of thedrum as it is carried by the conveyor for immersion into the dip coatcomposition and as it rises from the dip coat composition, while ittravels through a length for drainage and then the stuccoing hoodwherein the stucco is sprayed onto the surfaces wet with the dip coatcomposition and from the stuccoing section to a driving section tocomplete a cycle of separation which can be repeated continuously and anumber of times until the desired shell thickness has been built up onthe disposable pattern.

It will be apparent from the foregoing that the concepts described areadapted to enable the preparation of composite molds for metal castingswhich are capable of use in the production of castings of largedimension or of complicated shape and particularly precision castassemblies of air foil designs and wherein such molds can be produced inan efficient and economical manner for substantially mass productionwith a high yield of acceptable products.

While the invention has been described with reference to the preparationof molds of ceramic materials for the precision casting of super-alloysand high alloy steels, it will be understood that the concepts describedfor mold preparation may be adapted for the production of molds ofgraphite materials wherein the slip compositions are formulated ofgraphite flour and colloidal graphite while the stucco is selected ofsuitable particles of graphite to produce molds at least the innerportions of which are of graphitic composition.

It will be understood that changes may be made in the details ofarrangement, construction and operation without departing from thespirit of the invention, especially as defined in the following claims.

We claim:

1. In a process for the precision casting of metal parts of largedimension or complicated shape in molds, the steps of preparation of themold comprising separately preparing sections of the mold with edgeportions that interfit one with another to form a composite mold whichdefines the mold cavity, at least one section of which comprises acircular section with edge portions for interfitting with other sectionsof the composite mold, which includes the steps of forming patternelements of heat disposable material into a circular pattern assembly,rotating said circular pattern assembly while submerging at least aportion of the pattern in a fluid slip composition until all portions ofthe pattern assembly have been submerged uniformly to wet the patternassembly over its entire surface, exposing the pattern wet: with theslip coat composition to stucco for applying a stucco coat onto thesurface wet with the slip composition, repeating the alternating stepsof slip coating and stuccoing until a shell of the desired thickness hasbeen formed about the .heat disposable pattern, exposing the compositeto a temperature sufficient to effect removal of heat disposable patternmaterial to leave themold containing a cavity corresponding to thepattern assembly, molding the other sections of the mold of ceramicmaterial with interfitting edge portions for joinder in an interfittingrelation of the formed mold sections into a composite mold, joining theseparate sections of the mold along their interfitting edge portions andthen heating the assembly to cure the composite mold in which thecircular pattern comprises an air foil section having a plurality ofblades or vanes extending radially outwardly from a central hub sectionin circumferentially spaced apart relation.

2. A process as claimed in claim 1 in which the slip and stuccocompositions comprise ceramic materials.

3. In a process for the precision casting of metal parts of largedimension or complicated shape in molds, the steps of preparation of themold comprising separately preparing sections of the mold with edgeportions that interfit one with another to form a composite mold whichdefines the mold cavity, at least one section of which comprises acircular section with edge portions for interfitting with other sectionsof the composite mold, which includes the steps of forming patternelements of heat disposable material in a circular pattern assembly inwhich the elements extend outwardly radially from a central hub sectionin circumferentially spaced apart relation, mounting a plurality of saidcircular pattern assemblies in side-by-side axially aligned relationshipon a cylindrical support for rotational movement together with thesupport, rotating said circular pattern assembly while submerging atleast a portion of the pattern in a fluid slip composition until allportions of the pattern assembly have been submerged uniformly to wetthe pattern assembly over its entire surface, exposing the pattern wetwith slip coat composition to stucco for applying a stucco coating ontothe surface wet with the slip composition, repeating the alternatingsteps of slip coating and stuccoing until a shell of the desiredthickness has been formed about the heat disposable pattern, removingthe patterns and composite shell formed thereon from the cylindricalsupport, exposing the composite to a temperature sufficient to effectremoval of the heat disposable pattern material to leave the moldcontaining cavities corresponding to the pattern assembly, subdividingthe composite shell mold into separate sections, molding other sectionsof the mold of ceramic material with interfitting edge portions forjoinder in an interfitting relationship with the formed mold sectionsinto a composite mold, joining the separate sections of the mold alongtheir interfitting edges, and then heating the assembly to cure thecomposite mold.

4. A process as claimed in claim 3 in which the patterns are air foilpatterns and which includes a step of separating the air foil patternson the cylindrical support by a thin flexible disc shaped memberdimensioned to extend beyond the adjacent end portions of the pattern.

5. A process as claimed in claim 3 in which the alternating layers ofdip coat and stucco are applied by displacing the cylindrical supportwith the patterns mounted thereon to a position over a bath of the dipcoat composition, lowering the assembly for a distance to immerse thepatterns by an amount to wet the entire pattern assembly during rotationof the cylindrical support, raising the assembly from the bath after atleast one rotation in the bath, displacing the assembly to a positionoffset from the bath and sprinkling the assembly with stucco whilecontinuing to rotate the cylindrical support.

6. A process as claimed in claim 3 in which the alternating layers ofdip coat and stucco are applied by positioning a bath of dip coatcomposition into position beneath the rotating support, raising the bathinto position to immerse a portion of the pattern assembly sufficientlycompletely to wet the surfaces during rotation of the cylindricalsupport, lowering the bath after at least one revolution of the patternassembly while immersed in the bath, displacing the bath from beneaththe assembly and then sprinkling the assembly with stucco whilecontinuing to rotate the cylindrical support.

7. A process as claimed in claim 3 in which the patterns of the sectionsare separated one from the other on the cylindrical support by anannular rim which defines an annular void between the shell sections inthe composite mold and in which the shell sections are separated onefrom the other by removal of the portions of the shell radially of therim whereby the cavity formed by the rim portion defines dimensionallycontrolled end sections for interfitting the shell with others of theshell parts to form the completed mold.

8. A process as claimed in claim 7 in which the portion of the compositeshell radially of the annular rim is removed by grinding off thematerial as the composite mold is rotated until the cavity left by therim section is intersected.

9. A process as claimed in claim 1 in which others of the mold parts areformed by investment casting with the desired edge construction forjoinder one with the other.

10. A process as claimed in claim 1 in which the others of the moldparts are formed by compression molding the material with the desirededge construction for joinder one with the other.

1 1. A process as claimed in claim 1 which the mold parts are joined byan adhesive.

12. The pressure as claimed in claim 3 which includes the steps ofseparating the patterns and composite shell from the support,subdividing the composite shell and pattern into the separate sections,and then subjecting the sections to the pattern removal step.

13. A process as claimed in claim 1 in which the means joining theseparate sections of the mold comprise clamps to hold the partstogether.

14. A process as claimed in claim 13 in which the clamps are ceramicclamps.

15. A process as claimed in claim 11 in which the adhesive is a ceramicadhesive.

16. A process as claimed in claim 1 in which other sections of the moldcomprise metal sections cored for the passage of coolant therethrough.

* k IF UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,669,177 Dated June 13, 1972 Inventor(s) John galls et a] It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 8, line 9, change "pressure" to process Signed and sealed this17th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting; Officer Commissionerof Patents ORM PO-IOSO (10-69) USCOMM-DC 60376-3 69 u.s. GOVERNMENTPRINTING OFFICE: 1959 0-366-334

1. In a process for the precision casting of metal parts of largedimension or complicated shape in molds, the steps of preparation of themold comprising separately preparing sections of the mold with edgeportions that interfit one with another to form a composite mold whichdefines the mold cavity, at least one section of which comprises acircular section with edge portions for interfitting with other sectionsof the composite mold, which includes the steps of forming patternelements of heat disposable material into a circular pattern assembly,rotating said circular pattern assembly while submerging at least aportion of the pattern in a fluid slip composition until all portions ofthe pattern assembly have been submerged uniformly to wet the patternassembly over its entire surface, exposing the pattern wet with the slipcoat composition to stucco for applying a stucco coat onto the surfacewet with the slip composition, repeating the alternating steps of slipcoating and stuccoing until a shell of the desired thickness has beenformed about the heat disposable pattern, exposing the composite to atemperature sufficient to effect removal of heat disposable patternmaterial to leave the mold containing a cavity corresponding to thepattern assembly, molding the other sections of the mold of ceramicmaterial with interfitting edge portions for joinder in an interfittingrelation of the formed mold sections into a composite mold, joining theseparate sections of the mold along their interfitting edge portions andthen heating the assembly to cure the composite mold in which thecircular pattern comprises an air foil section having a plurality ofblades or vanes extending radially outwardly from a central hub sectionin circumferentially spaced apart relation.
 2. A process as claimed inclaim 1 in which the slip and stucco compositions comprise ceramicmaterials.
 3. In a process for the precision casting of metal parts oflarge dimension or complicated shape in molds, the steps of preparationof the mold comprising separately preparing sections of the mold withedge portions that interfit one with another to form a composite moldwhich defines the mold cavity, at least one section of which comprises acircular section with edge portions for interfitting wiTh other sectionsof the composite mold, which includes the steps of forming patternelements of heat disposable material in a circular pattern assembly inwhich the elements extend outwardly radially from a central hub sectionin circumferentially spaced apart relation, mounting a plurality of saidcircular pattern assemblies in side-by-side axially aligned relationshipon a cylindrical support for rotational movement together with thesupport, rotating said circular pattern assembly while submerging atleast a portion of the pattern in a fluid slip composition until allportions of the pattern assembly have been submerged uniformly to wetthe pattern assembly over its entire surface, exposing the pattern wetwith slip coat composition to stucco for applying a stucco coating ontothe surface wet with the slip composition, repeating the alternatingsteps of slip coating and stuccoing until a shell of the desiredthickness has been formed about the heat disposable pattern, removingthe patterns and composite shell formed thereon from the cylindricalsupport, exposing the composite to a temperature sufficient to effectremoval of the heat disposable pattern material to leave the moldcontaining cavities corresponding to the pattern assembly, subdividingthe composite shell mold into separate sections, molding other sectionsof the mold of ceramic material with interfitting edge portions forjoinder in an interfitting relationship with the formed mold sectionsinto a composite mold, joining the separate sections of the mold alongtheir interfitting edges, and then heating the assembly to cure thecomposite mold.
 4. A process as claimed in claim 3 in which the patternsare air foil patterns and which includes a step of separating the airfoil patterns on the cylindrical support by a thin flexible disc shapedmember dimensioned to extend beyond the adjacent end portions of thepattern.
 5. A process as claimed in claim 3 in which the alternatinglayers of dip coat and stucco are applied by displacing the cylindricalsupport with the patterns mounted thereon to a position over a bath ofthe dip coat composition, lowering the assembly for a distance toimmerse the patterns by an amount to wet the entire pattern assemblyduring rotation of the cylindrical support, raising the assembly fromthe bath after at least one rotation in the bath, displacing theassembly to a position offset from the bath and sprinkling the assemblywith stucco while continuing to rotate the cylindrical support.
 6. Aprocess as claimed in claim 3 in which the alternating layers of dipcoat and stucco are applied by positioning a bath of dip coatcomposition into position beneath the rotating support, raising the bathinto position to immerse a portion of the pattern assembly sufficientlycompletely to wet the surfaces during rotation of the cylindricalsupport, lowering the bath after at least one revolution of the patternassembly while immersed in the bath, displacing the bath from beneaththe assembly and then sprinkling the assembly with stucco whilecontinuing to rotate the cylindrical support.
 7. A process as claimed inclaim 3 in which the patterns of the sections are separated one from theother on the cylindrical support by an annular rim which defines anannular void between the shell sections in the composite mold and inwhich the shell sections are separated one from the other by removal ofthe portions of the shell radially of the rim whereby the cavity formedby the rim portion defines dimensionally controlled end sections forinterfitting the shell with others of the shell parts to form thecompleted mold.
 8. A process as claimed in claim 7 in which the portionof the composite shell radially of the annular rim is removed bygrinding off the material as the composite mold is rotated until thecavity left by the rim section is intersected.
 9. A process as claimedin claim 1 in which others of the mold parts are formed by investmentcasting with the desired edge construction for joindeR one with theother.
 10. A process as claimed in claim 1 in which the others of themold parts are formed by compression molding the material with thedesired edge construction for joinder one with the other.
 11. A processas claimed in claim 1 which the mold parts are joined by an adhesive.12. The pressure as claimed in claim 3 which includes the steps ofseparating the patterns and composite shell from the support,subdividing the composite shell and pattern into the separate sections,and then subjecting the sections to the pattern removal step.
 13. Aprocess as claimed in claim 1 in which the means joining the separatesections of the mold comprise clamps to hold the parts together.
 14. Aprocess as claimed in claim 13 in which the clamps are ceramic clamps.15. A process as claimed in claim 11 in which the adhesive is a ceramicadhesive.
 16. A process as claimed in claim 1 in which other sections ofthe mold comprise metal sections cored for the passage of coolanttherethrough.